For about US$5 just about anyone can build an alarm that will let you know that your engine's coolant temperature is getting high. It may alert you to a problem before your engine is ruined. This is a "pay attention" alarm, not a "you're dead" alarm. This design is applicable specifically to the OM603 and other engines that use a switch in the water jacket to turn on the auxiliary fan. Here is a picture of two such switches used by Mercedes.

Not all engines have such a switch but there is a work-around for those that don't. The work-around is more expensive (of course).

The alarm takes only two electronic components, an LED and a resistor, so it should be buildable even by folks with no electronics experience. (The LED has a tiny IC in it to make it blink about once a second. You can't adjust the blink rate, sorry.) I have deliberately kept this DIY as simple as possible; there are many ways to make it more complicated if you really want to. Although designed for my '87 300D Turbo (W124, OM603) it can be adapted to almost any car.

Parts list

1. Blinking red LED, Radio Shack part number 276-312 or equivalent, about US$2.
2. [optional] LED holder, Radio Shack part number 276-079 or equivalent, about US$1.50.
3. Resistor, 330 Ohms, 1/2 Watt, Radio Shack part number 271-1113 or equivalent, pack of five about US$1.
4. Wire, 18-24 gauge stranded.
5. Tape or shrink tubing for insulation.

The thermostat housing in the early 603 engines has two ports (tapped holes). One is a sensor for the Electronic Diesel System, the other is a dual switch. One part of the switch turns on the auxiliary electric fan in high speed at about 105C; the other part turns off the a/c at about 120C. The exact numbers vary from model to model. I figure that if the engine gets hot enough to turn on the auxiliary fan, I ought to know about it.

We are interested in the part of the switch that turns on the aux fan via a relay that lives in the fuse/relay box. Mercedes calls the switch "S25/5" and it is installed in the housing to which the upper radiator hose is attached. The wires that go to the three switches and sensors are shown in the following drawing.

S25/5 has three pins; one is by itself and the other two are together, numbered "1" and "2." All you need to do is to attach another wire to the connector for pin "2" and run it into the passenger compartment as the following drawing shows.

Now connect the blinking LED and the resistor together and then to something that will have 12 volts when the engine is running. Put the LED where it will point straight at your eyes when you are driving. It is not bright enough to be dangerous but bright enough that you will not miss it when it blinks at you. (The LED is made of clear plastic but the light is red.) Do not connect the LED directly to 12 volts or you will burn it out. That's what the resistor is for -- it limits the current that can flow through the LED.

I installed my LED in the instrument cluster next to the temperature gauge. This is not recommended for beginners as there isn't a lot of working room and you could damage something expensive. Here's what it looks like (I didn't use the Radio Shack LEDs).

The other LED is for the oil pressure alarm (DIY in another thread). I use the "Economy" gauge to monitor the vacuum level in the line to my transmission; the four-gauge cluster is from an '89 300E and is otherwise identical to the 300D gauges.

Modification for buzzer.
If you prefer a buzzer, just remove the LED and resistor and substitute a buzzer. Radio Shack has many kinds; just make sure you get one for 12 Volts. You can also add a switch for silencing the buzzer, just don't forget to turn the switch back on! You can use the LED to remind you that the buzzer is "OFF;" I'll let that be your job to design.

Modification for later engines.
Later model engines don't have a switch "S25/5." They use the climate control unit (CCU) to turn on the aux fan. If you have one of these cars, run your wire from pin 11 of connector X2, on the left side of the push button unit (PBU). An easy way to do this is to make a small loop from solid wire, put it over the pin, and then put the connector back on. The solid wire isn't very flexible so solder it to stranded wire. The rest of the alarm wiring is the same.

Modification for engines that don't have an aux fan switch.
A temperature-sensitive radiator switch can be installed in almost any car. There's an example in this web site:

This switch can also be used to control the aux fan directly and the thermostat allows you to set the temperature at which it comes on.

Conclusion
This could be a life-saver for those of us with alloy-headed engines. The only thing I might add is a "test" switch to turn the aux fan on manually, thus making sure that it and the LED actually work.